Moisture safety in CLT construction without weather protection

Introduction 

Wood has many advantages as a building material. High load-bearing capacity with low dead weight, flexible solutions with a high degree of prefabrication and a fast construction process, relatively good insulation capacity as a frame material, economic aspects and environmental benefits are some of the reasons for the increased use of wood in larger buildings. The construction sector's greater focus on resource efficiency means an increase in wood construction and especially construction with elements in cross-laminated timber (CLT) at height through increased load-bearing capacity.

Before these studies started, there was a perception in the industry, including internationally, that the compact wood mass of CLT panels should make it possible to build moisture-proof even when exposed to precipitation. However, there was no verified documentation on this.

Purpose and limits

The aim was to increase knowledge of how CLT structures, including joints, connections and attachment points, are affected by rainfall during construction, with a particular focus on moisture safety with regard to microbiological growth and critical moisture conditions. The long-term goal was to provide industry with a better basis for decision-making when planning CLT construction with good moisture safety. One limitation of the field project is that the results of remediation work were not included. The laboratory study was limited to clean water without exposure to dirt and rainwater.

Results

Two case studies with a total of four buildings were conducted, as well as interviews and literature review. Field measurements of moisture content, sampling of materials and analysis of mould growth were carried out during the structure erection period for CLT structures that were without comprehensive weather protection. Sampling was concentrated on points in the frame where the risk of moisture accumulation was high, such as joints, connections and attachment points, but also on open surfaces on the floor and at the bottom of walls / columns. Of a total of 200 analyzed measurement points, for these four buildings, half had sparse growth and about a third had moderate-rich growth. In this study, the growth found was often invisible to the naked eye, which meant that microbiological analysis was required to determine whether the material had been attacked or not. Based on the outcome of this study, it appears to be difficult or impossible to avoid the occurrence of microbial growth during construction with CLT without comprehensive weather protection. This meant that the Swedish National Board of Housing, Building and Planning's building rules and general regulations could not be followed in the first situation. Subsequently, extensive remediation work must have been done in all four objects, but the results of that work have not been included in this study. 

In the lab study, a laboratory test setup was developed to study mould growth under realistic and controlled climatic conditions after exposure to distilled water and spore suspension. In the experiments, small specimens of CLT structures were exposed to distilled water for 1 day or 1 week. During the development of the method, it was found that exposure for one day and then the possibility of open drying did not give rise to mold growth. However, growth did occur on surfaces that could not be dried immediately, e.g. at connection points. For specimens exposed for one week, mould growth occurred regardless of whether the surfaces could dry immediately or not. The conclusions apply primarily to the climates studied. The method needs further development, where other scenarios are studied and calibrated against samples exposed to outdoor air, dust, dirt and rainwater. 

Future research needs

Field evaluation of remediation measures especially in hard-to-reach areas.

The lab method needs to be further developed, where other scenarios are studied and calibrated against samples exposed to outdoor air, dust, dirt and rainwater.